This invention relates to a process for producing twin-free single-crystal bulk zinc selenide (ZnSe) suitable for use as a substrate for fabricating a blue-emitting semiconductor laser or LED by epitaxial growth of ZnSe compound in thin film.
Various methods are currently used to produce single-crystal bulk ZnSe from a melt and representative examples include: (1) an ordinary high-pressure melt process that uses a crucible having a fine tube in the growth start region as described in J. Cryst. Growth, 41 (1977) 103-108; (2) a process that performs growth in a Zn atmosphere as described in J. Cryst. Growth, 98 (1989) 302-308; (3) a high-pressure melt process that uses a crucible having a cone angle of 30 degrees as-described in J. Cryst. Growth, 117 (1992) 75-79; (4) a method that uses a "sealed hot-walled system" and a seed crystal as described in J. Cryst. Growth, 117 (1992) 80-84; and (5) a process that performs doping as described in J. Cryst. Growth, 86 (1988) 132-137.
In the production of single-crystal bulk ZnSe, the avoidance of twin formation is one of the major objectives to be attained. However, processes (1) and (2) do not take the formation of twins into account and only processes (3), (4) and (5) have succeeded in the growth of twin-free single-crystal bulk ZnSe. Nevertheless, process (3) is not very different from the ordinary high-pressure process and, what is more, the reproducibility of results is not taken into account. In process (4), a twin-free single crystal (having a smaller diameter than the constant-diameter portion of the growing crystal) is used as a seed; on the other hand, manganese (Mn) which may be considered as an impurity is added in process (5). Thus, the conventional processes under consideration have suffered from the following drawbacks: twins are formed in processes (1) and (2); process (3) does not have any means to prevent twin formation and the reproducibility of its results is not clearly established; (4) process involves great difficulty in obtaining a twin-free single crystal useful as a seed; and the effect that would be caused by the dopant Mn on the substrate characteristics is not clearly known with process (5).